HeartGold and SoulSilver take place in the Johto region as in
Pokémon Gold and Silver. After completing the main storyline,
players can also go to the Kanto region, similarly to in the
original Gold and Silver. Unlike the Pikachu in Pokémon
Yellow, HeartGold and SoulSilver will allow any Pokémon the
player owns to follow him or her in-game on the overworld. The
player may talk to the Pokémon, and occasionally it may pick up
certain secret items.